Process for manufacturing an air gap-insulated exhaust pipe

ABSTRACT

An air gap-insulated exhaust pipe with a sliding fit between two inner pipe sections in a middle area of the length of the exhaust pipe has a radial mounting of the inner pipe in the outer pipe which is provided in the area of a bend or beyond the bend, which joins the leg of the exhaust pipe in which the sliding fit is located. In terms of manufacturing technique, the radial mounting may be prepared before or after the bending of the double-pipe arrangement. The sliding fit may be prepared, even in the case of a straight exhaust pipe, manifold, or the like, by using a spacer sleeve and joint calibration of the pipe ends of the inner pipe sections, and the spacer sleeve burns off at the time the exhaust pipe is put into operation for the first time. As an alternative, the sliding fit may also be prepared by polygon calibration and rotating the pipe ends by half the pitch of the polygon.

This is a divisional of application Ser. No. 08/368,417 filed Jan. 4,1995.

FIELD OF THE INVENTION

The present invention pertains to an exhaust pipe, which is designed asan air gap-insulated, double-walled pipe with an outer pipe and an innerpipe, wherein two inner pipe sections connected via a sliding fit areprovided, and a radial mounting of the inner pipe in the outer pipe canbe provided at least at one point between the ends of the exhaust pipe,and the exhaust pipe may also have a first leg, a second leg, and a bendconnecting the two legs.

BACKGROUND OF THE INVENTION

Air gap-insulated, double-walled exhaust pipes have been increasinglyused especially in exhaust systems of motor vehicles, primarily for thearea between the internal combustion engine or its exhaust manifold,which collects the exhaust gases from a plurality of cylinders, and anexhaust gas catalytic converter; the latter normally has one or moreexhaust gas treatment bodies of large inner surface, through whichbodies exhaust gas can flow. The treatment bodies are coated with acatalytically active substance, wherein the exhaust gas treatment bodiesare mounted in a sheet-metal housing. Air gap-insulated exhaust pipesbring about a reduction in the release of heat by the exhaust gases tothe environment, so that the exhaust gas flows into the exhaust gascatalytic converter at a higher temperature than in the case of asingle-walled exhaust pipe. This is significant especially during thewarm-up period of the internal combustion engine, because the exhaustgas treatment body will thus rapidly reach its operating temperature. Inaddition, a relatively thin-walled inner pipe made of ahigh-temperature-resistant material and an outer pipe of greater wallthickness made of a less expensive material can advantageously be used.

Since the inner pipe of air gap-insulated exhaust pipes reachesconsiderably higher operating temperatures than the outer pipe, and itfrequently consists of a material different from that of the outer pipe,there will be differences in thermal expansion between the inner pipeand the outer pipe during operation. Depending on the length of the airgap-insulated exhaust pipes and the temperature difference between theinner pipe and the outer pipe, there will be differences in lengthbetween the inner pipe and the outer pipe, which may easily amount to afew mm. Such differences in length must be compensated, and theprovision of a sliding fit between the inner pipe and the outer pipe atone end of the exhaust pipe has hitherto been a common practice.However, particularly if the exhaust pipe has a bend, problems willarise in this simple design, because some areas of the inner pipe willcome very close to the outer pipe during heating, which entails the riskof noise generation due to metallic impacts.

The idea of rigidly connecting the inner pipe at both ends to the outerpipe and of providing a sliding fit between two inner pipe sectionssomewhere between the ends of the exhaust pipe has already beenproposed. However, such exhaust pipes have not yet been able to bemanufactured economically if a radial mounting of the inner pipe in theouter pipe is to be present somewhere between the ends of the exhaustpipe. Such a radial mounting is especially advantageous in the case ofrelatively long or bent exhaust pipes. In such cases, the onlypossibility has hitherto been to cut through the double-walled exhaustpipe, bent to its final shape, at right angles at a point, and tosubsequently prepare the sliding fit there between the two inner pipesections, as well as a radial mounting of the inner pipe in the outerpipe, after which the exhaust pipe was again welded together at thepoint of separation. Thus, the sliding fit and the radial mounting arepractically inherently located at the same point of the length of theexhaust pipe. The described manner of manufacture is cumbersome andexpensive.

SUMMARY AND OBJECTS OF THE INVENTION

The basic object of the present invention is to provide an airgap-insulated exhaust pipe of the type described in the introduction, inwhich both the differences in thermal expansion between the inner pipeand the outer pipe are compensated, and the generation of noise due tothe impaction of the inner pipe against the outer pipe is avoided withcertainty by using simple means.

The basic task of the present invention is accomplished by the slidingfit being provided at a first point, which is located in the first leg,and by a radial mounting of the inner pipe being provided at a secondpoint, which is located in the bend or in the second leg.

Contrary to the prior-art technology, a marked distance is thus providedbetween the sliding sit (first point) and the radial mounting (secondpoint). Sliding fits offer a certain resistance to the relative movementof the two partners of the sliding fit; according to the presentinvention, the radial mounting of the inner pipe in the outer pipe islocated at a point which prevents the inner pipe from yielding without arelative movement in the sliding fit, but it forces the sliding fit to"work." If the two legs of the exhaust pipe have different lengths, itis more favorable to provide the point containing the sliding fit in thelonger of the two legs.

It is pointed out that the exhaust pipe according to the presentinvention is usually provided only as a partial section of the entireexhaust pipe train; thus, one also could speak of an exhaust pipesection. It is also pointed out that the inner pipe is rigidly connectedto the outer pipe, although it is possible and sometimes even favorableto provide a sliding fit of the inner pipe relative to the outer pipe atleast at one end of the exhaust pipe. It is also pointed out that thepresent invention can also be embodied in an exhaust manifold or thelike, which may possibly have a shell design.

The radial mounting point is preferably located in the half of the bendadjacent to the second leg, or in the second leg in the vicinity of thebend. This arrangement ensures that the inner pipe is prevented mosteffectively from expanding as a unit without "working" of the slidingfit, and the difference in length in relation to the outer pipe isprevented most effectively from leading to an unsuitable reduction ofthe gap between the inner pipe and the outer pipe on the outside of theinner pipe bend.

The exhaust pipe according to the present invention is not limited tohaving only one bend as described. At least one additional bend may bepresent in the course of the first leg and/or of the second leg. If thisis so, the above explanations preferably pertain to the bend of theexhaust pipe that has the greatest change in direction.

A first preferred possibility of designing the radial mounting as avariant of the present invention is by means of wire knit fabric. Wireknit fabric offers the advantage of applying a sufficiently strongholding force, with a pre-tension, if desired, on the inner pipe and ofelastically compensating differences in radial thermal expansion betweenthe inner pipe and the outer pipe. The simplest possibility is acircular ring of wire knit fabric between the inner pipe and the outerpipe.

However, as a variant of the present invention, a plurality ofcircumferentially spaced wire knit fabric elements are preferablyprovided instead of one circular wire knit fabric ring. Depending on thegeometry of the course of the exhaust pipe and depending on the positionof the second point, this makes possible a simpler manufacture of theexhaust pipe, as will be explained below.

The wire knit fabric, be it, e.g., a wire knit fabric ring orcircumferentially spaced wire knit fabric elements, may be fastened tothe outer pipe and/or to the inner pipe, especially by welding. Thefastening of the wire knit fabric to the inner pipe or to the outer pipemay be performed during manufacture, prior to the fitting together andbending of the pipes, but it may also be performed after the fittingtogether and bending, especially in the case of a wire knit fabricpushed in subsequently as a radial mounting. As an alternative or inaddition to this, it is possible to fix the wire knit fabric in theouter pipe in the axial direction, again before or after fittingtogether and bending, preferably between two beads of the outer pipe,which are located at axially spaced locations from one another.

A second preferred possibility of designing the radial mounting as avariant of the present invention is to provide it by means of aplurality of inwardly extending impressions of the outer pipe, which arelocated at circumferentially spaced locations from one another. Theseimpressions can be prepared particularly efficiently and after fittingtogether and bending. The impressions should best extend inwardly to theextent that the inner pipe will be in contact with the impressions fromthe inside already in the cold state, so that no noise will be generatedthere even in the cold state.

In a preferred embodiment of the present invention, the impressions arespaced circumferentially so far from one another that circumferencesections of sufficient length are free for elastic bulging during anincrease in temperature at the second point on the inner pipe. Thematerial of the inner pipe has a markedly lower strength at theoperating temperature than in the cold state. It is therefore preferablefor the impressions not to have only point contact with the inner pipe,but over a circumferential length corresponding to at least 30 degrees.In the circumference sections described between the contact(s) with theimpressions, the inner pipe is able to elastically bulge out during anincrease in temperature, without plastic deformation with the risk of asubsequently looser seating in the outer pipe taking place.

In a variant of the present invention, it is possible to manufacture thetwo inner pipe sections from different materials and/or with differentwall thicknesses. Since the stresses on the inner pipe may be differentover the length of the exhaust pipe, depending on the geometry of thepipe, it may be advantageous to provide an especially high-valuematerial and/or a great wall thickness only where this is reallynecessary.

On the whole, austenitic, highly heat-resistant steels are usually usedfor the inner pipe, and less expensive, ferritic steels are used for theouter pipe.

Another object of the present invention is a process for manufacturingthe exhaust pipe according to the present invention with bend and legs,characterized in that

(a) the inner pipe, which has the two inner pipe sections connected viathe sliding fit at the first point, is inserted into the outer pipe,while the radial mounting is prepared at the same time at the secondpoint,

(b) the space between the outer pipe and the inner pipe is filled withan essentially incompressible medium,

(c) the bend is prepared by bending the double pipe arrangement, and

(d) the medium is removed from the space between the outer pipe and theinner pipe.

As an alternative, the present invention also pertains to a process formanufacturing the exhaust pipe, characterized in that

(a) the inner pipe, which has the two inner pipe sections connected viathe sliding fit at the first point, is inserted into the outer pipe,

(b) the space between the outer pipe and the inner pipe is filled withan essentially incompressible medium,

(c) the bend is prepared by bending the double pipe arrangement,

(d) the medium is removed from the space between the outer pipe and theinner pipe, and

(e) the radial mounting is prepared at the second point.

Steel shot is preferably used as the incompressible medium.

As was described, the radial mounting is prepared in the aforementionedtwo processes by wire knit fabric, be it a wire knit fabric ring orcircumferentially spaced elements of wire knit fabric, or by means ofinwardly extending impressions of the outer pipe. Depending on theposition of the second point in the exhaust pipe, there are, however,pipe sections in which wire knit fabric can subsequently be pushed inwith difficulty at best after the bending of the double pipearrangement. In the case of a radial mounting formed by impressions, itis often more efficient to prepare the radial mounting after the bendingof the double pipe arrangement.

A radial mounting formed by impressions also offers the advantage thatany eccentricity between the inner pipe and the outer pipe, which may bedue to manufacturing tolerances of the inner pipe and the outer pipe aswell as to the bending process, can be corrected by the impressions.However, it is also possible to intentionally provide impressions ofdifferent depth over the circumference and thus to bring about anintended eccentricity, especially in order to create more space betweenthe inner pipe and the outer pipe in a defined radial direction of theradial mounting. As a result, more free space is created in a defineddirection for temperature-dependent displacements of the inner pipe nextto the radial mounting point.

It is emphasized that in addition to the radial mounting described, theexhaust pipe according to the present invention may also have anotherradial mounting or even a plurality of radial mountings of the innerpipe in the outer pipe at the second point described.

Special variants of the present invention include especially twovariants of a sliding fit, via which the two inner pipe sections of theinner pipe are displaceably connected to one another, which variants areindependent per se. The exhaust pipe may be designed with or withoutbend, i.e., as a straight exhaust pipe. As is stated in the claims, bothvariants of the present invention pertain not only to air-insulated,double-walled exhaust pipes, but the special variants of sliding fitsmay also be used in manifolds with or without shell design, plug-typepipe connections or the like. However, the variants of the sliding fitwill be discussed here in connection with an exhaust pipe.

The first variant of the sliding fit especially pertains to the use of aspacer sleeve for mounting the inner pipe or for fitting together theinner pipe sections, wherein the spacer sleeve is inserted especiallyinto the calibrated end area of one of the inner pipe sections, whichend area forms the joining gap, until it reaches a defined, first axialend stop, after which the other inner pipe section is in turn insertedinto a second, defined axial end stop of the spacer sleeve. Aftermounting of the sliding fit, the entire inner pipe is arranged in theouter pipe (or in a one-part or multipart shell), and the spacer sleevemay have radially outwardly projecting centering cams, which center thespacer sleeve, including the inner pipe area of the spacer sleeve, inthe outer pipe, in order to provide an accurate, simple seating for theinner pipe before the latter is finally installed at another axial pointin the outer pipe.

The spacer sleeve is made of a combustible material, such aspolyethylene or polystyrene, which has a combustion temperature that islower than the exhaust gas temperature of an internal combustion engineor the like. Thus, when the finally installed exhaust pipe is put intooperation for the first time, the spacer sleeve will burn withoutresidue, thereby releasing the exact sliding fit set before by thespacer sleeve. Thus, if sand (or another filling medium) is used forbending a bent exhaust pipe, this sand cannot enter the joining gap ofthe inner pipe sections, because this gap is occupied by the spacersleeve until the sand is also removed. Contamination of the area of thesliding fit by filling medium is consequently not possible in any way.

The spacer sleeve provides for a forced positioning of the inner pipesections in the outer pipe in the installed state. By using a spacersleeve, both inner pipe sections are calibrated together in the plug-inarea.

By using the aforementioned spacer sleeve, not only are the depth ofinsertion and the path of sliding defined and a forced positionprovided, but it is also possible, in particular, to reduce thetolerance of the gap dimension by at least 50% compared with individualprocessing of the two parts. In particular, it is possible to reduce thegap dimension between the pipe and the sleeve and between the inner pipesections in the insertion area concerning diameter and layout toleranceby joint calibration. The concentricity of the gas-carrying inner pipewith the outer pipe during the phase of mounting is guaranteed by thecentering cams of the spacer sleeve.

The second variant of the sliding fit pertains to a special calibrationprocess for the circumference of the inner pipe sections in the end areaof the plug-type and sliding connection. In particular, the two endareas are provided individually with a so-called polygon calibration,namely, by means of special calibrating mandrels or calibratingexpanding segments, which create (in the front view of the inner pipe) apolygon by flattened areas of the previously round diameter, while roundareas remain on the circumference between the flattened areas. Theflattened areas, preferably three per inner pipe section, are especiallydistributed at equal distance on the circumference.

The inner pipe sections, individually calibrated polygonally, are nowrotated circumferentially in relation to one another after they havebeen fitted together, until axial line contact is achieved between theparts. The circumferential rotation preferably corresponds to half apolygon pitch.

Thus, a defined shaping of the inner pipe sections is achieved accordingto the present invention, which prevents a relative movement of the twopipe ends of the plug-type connection in the radial direction by atleast three axial line contacts between the parts, but it continues topermit a displacement of the parts in the axial direction, so that norattling noises can develop during operation, not even on cold start ofa vehicle. The positions of the two pipes in relation to one another arefixed despite the necessary gap. The outlays of the two pipe ends aredimensioned such that even different radial thermal expansions, whichmay occur, e.g., when different materials are used, can be compensated,without leading to blockage of the sliding fit. Thus, quasi a "specialout-of-roundness" is created by the present invention in order toprevent rattling noises in a simple manner, even though it is notnecessary to impose excessive requirements on the manufacturingtolerances of the individual parts.

The various features of novelty which characterize the invention arepointed out with particularity in the claims annexed to and forming apart of this disclosure. For a better understanding of the invention,its operating advantages and specific objects attained by its uses,reference is made to the accompanying drawings and descriptive matter inwhich preferred embodiments of the invention are illustrated.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1 is a partially cut-away view of a double-walled exhaust pipe;

FIG. 2 is a cross section of the exhaust pipe according to FIG. 1 alongline II--II;

FIG. 3 is a partially cut-way partial view of a modified embodiment ofthe exhaust pipe according to FIG. 1;

FIG. 4 is a cross section of the exhaust pipe according to FIG. 3 alongline IV--IV;

FIG. 5 is a section similar to FIG. 1 in the area of a sliding fit of aspecial design;

FIG. 6 is a front view of FIG. 5, with the outer pipe omitted;

FIG. 7 is a cross section of FIG. 6 along line I--I;

FIG. 8 is a front view in the joined state of another sliding fitsimilar to that shown in FIG. 6;

FIG. 9 is a cross section of FIG. 8 along line III--III;

FIG. 10 is a front view of the sliding fit according to FIG. 8 afterrotation or in the mounted state; and

FIG. 11 is a cross section of FIG. 10 along line V--V, similar to FIG.9.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The exhaust pipe 2 shown in FIG. 1 has an outer pipe 4 and an inner pipe6, which is essentially concentric to the outer pipe, as its principalcomponents. The diameter of the outer pipe 4 is reduced at both ends 8,10 of the exhaust pipe 2 to the extent that it is in contact with theinner pipe 6, as can be recognized in the lower right part of FIG. 1.The outer pipe 4 and the inner pipe 6 are rigidly connected to oneanother, e.g., by welding, at the two ends 8, 10.

The inner pipe consists of a first inner pipe section 12 and a secondinner pipe section 14, which are connected to one another via a slidingfit 16 at a first point, which will be described more specificallybelow. To form the sliding fit 16, the diameter of one of the two innerpipe sections 12, 14 is expanded in its end area there to the extentthat the end area of the other inner pipe section at that location fitsin with a relatively narrow, rattle-free seating, which does, however,permit a relative axial displacement of the two inner pipe sections inthe sliding fit 16. Other details of design variants of the sliding fitwill be described below.

Concerning the geometric shape, the exhaust pipe 2 is comprised of afirst leg 18, a bend 20 joining the first leg 18, and a second leg 22joining the bend 20. The first leg 18 has a considerably greater lengththan the second leg 22. Due to the bend 20, the exhaust pipe 2 has arelatively great change in direction there, which equals ca. 70° (angleW1) in the exemplary embodiment shown. The drawing plane of FIG. 1 isplaced such that the longitudinal central axis of the second leg 22, thebent longitudinal central axis of the bend 20, and the longitudinalcentral axis of a partial area of the first leg 18 joining the bend 20are located in the plane of the drawing.

The first leg 18 is not continuously straight, but it has another bend24, which defines a much smaller change in direction than theabove-described bend 20. The bent longitudinal central axis of the otherbend 24 may, but does not have to, be located in the drawing plane ofFIG. 1, so that the partial area of the first leg 18 located between theother bend 24 and the exhaust pipe end 8 may be located in the drawingplane or it may be led out of the drawing plane to the rear or to thefront.

At a second point, which is located rather close to the end of the bendfacing the second leg 22, a radial mounting 26 of the inner pipe 6 inthe outer pipe 4 is provided. In the exemplary embodiment shown in FIGS.1 and 2, the radial mounting 26 consists of three circumferentiallydistributed wire knit fabric elements 28, which are spaced from oneanother. In the exemplary embodiment shown, the wire knit fabricelements 28 have a circumferential length of 30° each. It would also bepossible to provide a different number of wire knit fabric elements or acircular wire knit fabric ring.

The position of the sliding fit 16 in the first leg 18 is not critical.The sliding fit is usually provided approximately in the middle area ofthe length of the first leg 18.

When hot exhaust gas flows through the air gap-insulated, double-walledexhaust pipe 2, the inner pipe 6 undergoes a greater longitudinalexpansion than the outer pipe 4, and a considerably larger portion ofthe differences in longitudinal expansion affects the first leg 18. Thedifference in longitudinal expansion is absorbed by the sliding fit 16there, i.e., the two inner pipe sections 12, 14 are pushed together moreby a certain amount there. The sliding fit 16 cannot be pushed togetherfurther without resistance. However, the radial mounting 26 at thesecond point described ensures that the sliding fit 16 does indeed"work" by exerting a pushing-together action during an increase in thetemperature of the inner pipe 6, and the inner pipe 6 does not simplyyield more to the outside of the bend 20 due to a local displacement,because the radial mounting 26 intentionally prevents precisely thisyielding movement of the change in length originating from the first leg18. A difference in longitudinal expansion between the outer pipe 4 andthe inner pipe 6 is, of course, also generated in the second leg 22, butthis difference is smaller because of the shorter length of the secondleg 22 compared with that of the first leg 18. This difference inlongitudinal expansion is absorbed by the inner pipe 6 coming somewhatcloser to the outer pipe 4 approximately in the area 29 of the outsideof the bend 20. The annular gap between the outer pipe 4 and the innerpipe 6 is large enough to absorb the displacement taking place there.Something similar takes place approximately in the area 30 of theoutside of the other bend 24 as the effect of the difference in thelongitudinal expansion between the inner pipe 6 and the outer pipe 4 inthe partial area of the first leg 18 adjacent to the exhaust pipe end 8.Even if this partial area has a considerable length, the effects areless marked here, because the other bend 24 defines a smaller change indirection than the bend 20.

The radial mounting 26 may also be displaced farther to the left intothe bend 20, and even beyond its apex, or it also could be positioned inthe second leg 22, but preferably not far from the bend 20.

FIGS. 3 and 4 show modified embodiments of the radial mounting 26, inwhich only the differences from the first embodiment described aredescribed. Instead of the wire knit fabric elements 28, a plurality ofimpressions 32, which are spaced from one another and distributed overthe circumference of the exhaust pipe 2, are prepared on the outer pipe4. The impressions 32 extend inwardly to the extent that there is arattle-free mounting contact between the inner pipe 6 and the outer pipe4 in the cold state of the exhaust pipe. In the exemplary embodimentshown, the circumferential length of the impressions, which is incontact with the inner pipe 6, is ca. 90°; two diametrically opposedimpressions 32 are provided. It would also be possible to provide agreater number of impressions 32 distributed over the circumference andlocated at spaced locations from one another in the circumferentialdirection. However, free circumferential sections 34 of the inner pipe6, which are long enough in the circumferential direction to permit theinner pipe 6 to bulge out elastically during an increase in temperature,are preferably left between the impressions 32, which are in contactwith the inner pipe 6. This prevents the inner pipe 6 from undergoingplastic deformation during an increase in temperature in the radialmounting 26. The consequence of a plastic deformation taking place therewould be that the inner pipe 6 would no longer be seated tightly in theradial mounting 26 during the subsequent cooling.

It is pointed out that the exhaust pipe described may also haveadditional bends, e.g., another bend even with a relatively great changein direction in the first leg 18 rather close to the end 8. If the pipelength between this additional bend and the end 8 is not too great, thefastening of the inner pipe 6 and of the outer pipe 4 to one another inthe area of the end 8 brings about a fixation which prevents thedescribed undesired yielding movements of the inner pipe 6. Especiallyif the additional bend defines a great change in direction, and the pipelength between this additional bend and the end 8 is also considerableat the same time, an additional radial mounting 26 analogous to theradial mounting described may be provided in that area.

The exhaust pipes 2 described so far can be manufactured by insertingthe inner pipe 6 already provided with the sliding fit 16 into the outerpipe 4 and establishing the firm connection between the inner pipe 6 andthe outer pipe 4 at one of the two ends 8, 10. Then, e.g., sand isintroduced into the annular gap space between the inner pipe 6 and theouter pipe 4 over the entire length. This double pipe arrangement canthen be bent according to a common, simple technology to prepare anydesired bend 20, 24, etc. The sand is then released from the annular gapspace via the yet unconnected end 10. The firm connection between theinner pipe 6 and the outer pipe 4 is finally prepared at the end 10.

The radial mounting 26 may optionally be prepared before or after thebending process. For the first possibility, the three wire knit fabricelements 28 are fastened, e.g., to the inner pipe 6 by resistancewelding before this inner pipe 6 is inserted into the outer pipe 4. Thesand to be filled in can pass through the radial mounting point throughthe free circumferential areas between the wire knit fabric elements 28.The situation is similar in the case of the alternative radial mounting26 by means of impressions 32 if these are prepared prior to the bendingof the double pipe arrangement. However, the preparation of the radialmounting 26 with the impressions 32 is especially suitable for thesecond possibility, i.e., preparation after bending. However, the secondpossibility can also be carried out with a wire knit fabric ring or withwire knit fabric elements 28, e.g., by pushing axially into the annularspace after bending and removal of the sand. Axial fixation of the wireknit fabric may be performed, e.g., by circular impressions on bothsides next to the wire knit fabric, by resistance welding to the outerpipe 4 or the like.

The radial mounting 26 is usually prepared with such a strong pressingforce on the inner pipe 6 that the inner pipe 6 can still be displacedin the axial direction there in relation to the outer pipe 4.Excessively strong pressing force is disadvantageous, because the innerpipe 6 would thus reach the range of plastic deformation in the area ofthe radial mounting 26 too easily during an increase in temperature.

FIGS. 5 through 7 show a special embodiment of a sliding fit 16 betweenthe inner pipe sections 12, 14, which are concentrically embedded in theouter pipe 4 of the exhaust pipe 2 and are also centered.

In particular, a spacer sleeve 40 is used during the assembly of thejointly calibrated inner pipe sections 12, 14 by fitting together; thisspacer sleeve has a circular design, comprised of a combustiblematerial, preferably a plastic, and it is burned by the heat of theexhaust gas at the time the exhaust pipe is put into operation for thefirst time after complete installation, so that it releases afunctionally and dimensionally optimized sliding fit 16. The spacersleeve 40 is consequently needed only during the phase of manufacture.

The spacer sleeve has circumferential shoulders at its axial ends. Oneof the circumferential shoulders of the spacer sleeve, the right-handone according to the drawing, is a radially inwardly pointingcircumferential projection, and it forms a first axial end stop 41 forthe pipe end of the second inner pipe section 14. The othercircumferential shoulder of the spacer sleeve 40, the left-hand oneaccording to the drawing, is a radially outwardly pointingcircumferential projection, and it forms a second axial end stop 42 forthe pipe end of the first inner pipe section 12. The axial distancebetween the two axial end stops 41, 42 determines the depth of insertiont of the second inner pipe section 14 in the first inner pipe section12, and consequently also the path of sliding s at a predetermined axiallength of a calibration of the inner pipe section 12, as can bedetermined especially from FIG. 5.

The spacer sleeve 40 also has, radially on the outside to the secondaxial end stop 42, four centering cams 43, which are uniformlydistributed over the circumference, and the spacer sleeve as a whole isdesigned as a one-piece spacer sleeve, i.e., the centering cams 43 andthe axial end stops 41, 42 are integrated components of the sleeve. Inparticular, three centering cams 43 (contrary to the exemplaryembodiment shown) are used, which, statically determined, make possiblean accurate contact with the inner circumference of the outer pipe 4.

A defined shaping as well as the use of a spacer sleeve 40 thusguarantee that rattling noises and function-impairing disturbancescannot occur in the pulsating gas flow and during operation, because thejoining gap can be adjusted depending on the temperature and expansion.

FIGS. 8 through 11 show another variant of the sliding fit, which alsoprevents rattling noises or blockage, as described above, from occurringby simple means.

In particular, a defined shaping is used, which is formed by polygoncalibration on the circumference of the end areas of the two inner pipesections 12, 14.

During the calibration of the inner pipe sections 12, 14 in theexemplary embodiment shown in FIGS. 8 through 11 by means of sixexpanding segments, a surface of at least 0.3 mm is ground on everyother segment in the outer radius. A circular shape with three flattenedareas 44 offset by 120° each is formed at the pipe ends.

In the joined state according to FIGS. 8 and 9, the flattened areas 44of the two pipe ends are located congruently one above another andrelease the joining gap.

The inner pipe sections 12, 14 are then rotated to the installed stateaccording to FIGS. 10 and 11 by half the pitch angle of the polygon,here 60°, in relation to one another, and three line contacts, whichreliably prevent rattling and equally guarantee the axialdisplaceability of the individual parts, are formed. The inner pipesection 14, which is heated during operation, is able to radially expandinto the free circumferential spaces 45, without causing blockage of thesliding fit.

While specific embodiments of the invention have been shown anddescribed in detail to illustrate the application of the principles ofthe invention, it will be understood that the invention may be embodiedotherwise without departing from such principles.

What is claimed is:
 1. A process for manufacturing an air gap-insulated,double-walled exhaust pipe, as part of an air gap-insulated exhaust pipeassembly, the process comprising the steps of:providing an outer pipe;providing a gas-carrying inner pipe, which has a first inner pipesection and a second inner pipe section; connecting said first innerpipe section and said second inner pipe section to one another via asliding fit; and maintaining a space between said two inner pipesections to preserve said sliding fit during subsequent formation ofsaid air gap-insulated exhaust pipe assembly.
 2. The process accordingto claim 1, wherein:said outer pipe and said two inner pipe sectionsconnected via said sliding fit define said air gap-insulated exhaustpipe assembly, said assembly having a first leg, a second leg, and abend connecting the two legs, a radial mounting for mounting said twoinner pipe sections connected via said sliding fit in said outer pipe atleast at one point between ends of said exhaust pipe assembly, saidsliding fit connection being provided at a first point, which is locatedin said first leg, and said radial mounting being provided at a secondpoint, the process further comprising the steps of:(a) inserting saidtwo inner pipe sections connected via said sliding fit into said outerpipe at a location adjacent to said the first point; (b) filling thespace between said outer pipe and said inner pipe with an essentiallyincompressible medium; (c) preparing said bend by bending said doublepipe arrangement; (d) removing the medium from the space between saidouter pipe and said inner pipe; and (e) preparing said radial mountingat said second point.
 3. The process for manufacturing a double-walledexhaust pipe according to claim 2, wherein said radial mounting isprepared at substantially the same time as said step of inserting saidinner pipe.
 4. The process for manufacturing a double-walled exhaustpipe according to claim 2, wherein said radial mounting is preparedsubsequently to said step of removing the medium.
 5. The processaccording to claim 1, further comprising the steps ofusing a spacersleeve arranged between said first inner pipe section and said secondinner pipe section for maintaining said space during mounting of saidinner pipe sections, said inner pipe sections being aligned axially toone another and have a joining gap defining said sliding fit, saidspacer being disposed in said joining gap; and subsequently burning saidspacer by heat of the exhaust gas when the said exhaust pipe is put intooperation for the first time, thus releasing said sliding fit.
 6. Theprocess in accordance with claim 5, wherein a depth of insertion (t)and/or the path of sliding (s) of said second inner pipe section in saidfirst inner pipe section is determined by said spacer sleeve.
 7. Theprocess in accordance with claim 5, wherein a gap dimension of a joininggap in terms of a diameter and layout tolerance of said inner pipesections is determined by said spacer sleeve by joint calibration. 8.The process in accordance with claim 5, wherein said inner pipe iscentered with said outer pipe by said spacer sleeve.
 9. The process inaccordance with claim 7, wherein said spacer sleeve is inserted into acalibrated end area of said first inner pipe section forming the joininggap to a defined first axial end stop, and an end area of said secondinner pipe section is subsequently inserted up to a defined second axialend stop.
 10. The process in accordance with claim 9, wherein said innerpipe sections, fitted together, including the said spacer sleeve, areinserted into said outer pipe, and said inner pipe is centered in thesaid outer pipe by radially outer centering cams of the said spacersleeve, which are distributed on the circumference.
 11. The processaccording to claim 1, further comprising the steps of:preparing saidsliding fit by polygon calibration on a circumference of said inner pipesections in an end area of a plug-type sliding connection; andmaintaining said space with individually polygonally calibrated innerpipe sections and rotating said sections in relation to one anotherafter having been fitted together, until an axial line contact is formedbetween the end areas of said inner pipe sections.
 12. The process inaccordance with claim 11, wherein said individually polygonallycalibrated inner pipe sections are rotated in relation to one another byhalf of a pitch angle of the polygon calibration after fitting together.13. The process in accordance with claim 11, wherein the polygoncalibration is prepared by uniformly distributed flattened areas,especially three said flattened areas per said pipe section, on thecircumference of the pipe.
 14. The process in accordance with claim 11,wherein the polygon calibration is performed by means of fixed oradjustable calibrating mandrels.
 15. The process in accordance withclaim 11, wherein the polygon calibration is performed by means ofexpanding segments.
 16. The process in accordance with claim 15, whereinthe polygon calibration is performed by means of six expanding segments,with a surface of at least 0.3 mm ground on every other expandingsegment in an outer radius thereof.
 17. A process for manufacturing anair gap-insulated, double-walled exhaust pipe, as part of an airgap-insulated exhaust pipe assembly, the process comprising the stepsof:providing an outer pipe; providing a gas-carrying inner pipe, whichhas a first inner pipe section and a second inner pipe section;connecting said first inner pipe section and said second inner pipesection to one another via a sliding fit; and maintaining a spacebetween said two inner pipe sections and said outer pipe duringsubsequent formation of said air gap-insulated exhaust pipe assembly.18. The process according to claim 17, wherein:wherein a radial mountingis used for mounting said two inner pipe sections connected via saidsliding fit in said outer pipe at least at one point between ends ofsaid exhaust pipe assembly, the process further comprising the stepsof:(a) inserting said two inner pipe sections connected via said slidingfit into said outer pipe at a location adjacent to said the first point;(b) filling the space between said outer pipe and said inner pipe withan essentially incompressible medium; (c) preparing said bend by bendingsaid double pipe arrangement; (d) removing the medium from the spacebetween said outer pipe and said inner pipe; and (e) preparing saidradial mounting at said second point.
 19. The process according to claim17, further comprising the steps of:using a spacer sleeve arrangedbetween said first inner pipe section and said second inner pipe sectionfor maintaining a space during mounting for preserving said sliding fitof said inner pipe sections, said inner pipe sections being alignedaxially to one another and have a joining gap defining said sliding fit,said spacer being disposed in said joining gap; and subsequently burningsaid spacer by heat of the exhaust gas when the said exhaust pipe is putinto operation for the first time, thus releasing said sliding fit. 20.The process according to claim 17, further comprising the stepsof:preparing said sliding fit by polygon calibration on a circumferenceof said inner pipe sections in an end area of a plug-type slidingconnection; and maintaining a space during mounting for preserving saidsliding fit of said inner pipe sections with individually polygonallycalibrated inner pipe sections and rotating said sections in relation toone another after having been fitted together, until an axial linecontact is formed between the end areas of said inner pipe sections.